In commonly owned U.S. application Ser. No. 53,275, filed June 29, 1979 by three of us (Guido Busacca, Vincenzo Meli and Arcangelo Passaglia), now U.S. Pat. No. 4,247,828, there has been described a mechanism for the purpose set forth in which a tuning piston is rigid with a yoke spacedly surrounding a nonrotatable following ring. This ring hugging an eccentric portion of a rotating crankshaft through the intermediary of a ball bearing is suspended between two flexible wires or blades which are anchored to the piston and to an extension thereof on the opposite side of the yoke. This arrangement enables the piston to be linearly guided while allowing the ring to follow the lateral excursions of the eccentric crankshaft portion.